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<html lang="en">

<head>
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    <title>Document</title>
    <script src="../jquery-3.3.1.min.js"></script>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        .bi {
            width: 90%;
            height: 600px;
            margin: 0 auto;
            top: 50px;
            background-color: black;
        }
        
        ul {
            width: 90%;
            height: 600px;
            background-color: black;
        }
        
        ul>li {
            width: 29%;
            height: 250px;
            float: left;
            margin-left: 3%;
            margin-top: 20px;
            background-color: white;
            list-style: none;
            opacity: 0.5;
        }
    </style>
</head>

<body>
    <div>
        <ul>
            <li></li>
            <li></li>
            <li></li>
            <li></li>
            <li></li>
            <li></li>
        </ul>
    </div>
    <script>
        $(function() {
            $("li").hover(function() {
                $(this).css("opacity", "1.0");
            }, function() {
                $(this).css("opacity", "0.5");
            })
        });
    </script>
</body>

</html>